"MineAssemble is a tiny bootable Minecraft clone written partly in x86 assembly. I made it first and foremost because a university assignment required me to implement a game in assembly for a computer systems course. Because I had never implemented anything more complex than a 'Hello World' bootloader before, I decided I wanted to learn about writing my own kernel code at the same time. Note that the goal of this project was not to write highly efficient hand-optimized assembly code, but rather to have fun and write code that balances readability and speed. This is primarily accomplished by proper commenting and consistent code structuring." Just cool.

Actually, in this specific case, Elder Scroll games have shown for decades how large game maps can be used without excessive RAM usage. Simply put:
* Take a huge world map
* Slice it into smaller chunks
* Only keep the current chunk and its nearest neighbours in memory
* Profit !

I would be surprised if Minecraft didn't do something similar on the inside, keeping the whole world map loaded all the time sounds like a needlessly inefficient way to go about it.